1. Field of the Invention
This invention relates to an improved high performance honeycomb comprising thermoplastic binder having a melt point of from 120° C. to 350° C., methods for making the honeycomb, and articles comprising the honeycomb; the honeycomb is made with a paper that allows rapid impregnation of the honeycomb by structural thermoset resins while retarding excessive impregnation of node-line adhesives during manufacture.
2. Description of Related Art
Paper-based honeycomb is typically formed by (1) applying adhesive resin to sheets of paper along predetermined lines, called node lines, (2) adhering several sheets of paper along these node lines to form a stack, with the node lines of each sheet offset to the adjacent sheets, (3) expanding the stack to form a honeycomb having defined cell walls, (4) impregnating the cell walls of the honeycomb with structural resin by submerging the honeycomb in a liquid resin, and (5) curing the resin with heat. U.S. Pat. No. 5,137,768 to Lin; U.S. Pat. No. 5,789,059 to Nomoto; and U.S. Pat. No. 6,544,622 to Nomoto; disclose honeycombs made from sheets made from high modulus para-aramid materials. These honeycombs are highly prized for structural applications due to their high stiffness and high strength to weight ratio. Generally these honeycombs are made from papers comprising para-aramid fibers, pulp, and/or other fibrous materials plus a binder.
U.S. Pat. Nos. 6,551,456 and 6,458,244 to Wang et al. and Japanese Patent Application Publication 61-58,193 to Nishimura et al., disclose papers made from aramid fibers combined with polyester fibers. It has been found that these papers have a very open or porous structure, allowing rapid impregnation of thermoset structural resins.
Unfortunately, if these aramid/polyester papers are used for honeycomb, the high porosity to resins can also allow rapid penetration of the node line adhesive resin through the paper. It is highly desired that the adhesive, when printed or applied to the surface of the paper, remain substantially on the surface of the paper and not penetrate through the paper to the opposite surface. Otherwise, the paper sheets are simply glued together and are impossible to expand into a uniform honeycomb structure. This problem is particularly critical for thin papers having a thickness of 75 micrometers or less that are highly desired for high-performance lightweight aircraft honeycombs.
Typically, the application or printing of the adhesive nodes lines is a relatively fast process, while impregnation of the structural resin is a somewhat slower process. Therefore what is needed is a honeycomb made from a paper that has properties that can control the rate of impregnation by the adhesive resin while maintaining overall good impregnation of structural resin.